Most database management systems available today are based on the relational database model. This model is designed around the mathematical concepts of relational algebra and it is based upon the following operations that are identified in relational algebra:
SELECTION - This operation selects a record or records from a table based on a specified condition.
PROJECTION - This operation returns a column or columns from a table based on some condition.
JOIN - This operation pastes two or more tables together, side by side. Each table must have a common column before a JOIN can work.
UNION - This operation combines two tables to produce a set of all records found in both tables. Each table must have compatible columns before a UNION can work
DIFFERENCE - This operation tells you what records are unique to one table when two tables are compared. Each table must have identical columns before a DIFFERENCE can work
INTERSECTION - This operation tells you what records are common to two or more tables when they are compared.
In a relational database, data is perceived to exist in one or more two dimensional tables. These tables are made up of rows and columns, where each record (row) is divided into fields (columns) which contain individual pieces of information. The real form of the data storage may be different, but visualizing it as a collection of two-dimensional tables makes it easier to describe and understand them.